Method of distilling a volatile constituent from a liquid mixture

ABSTRACT

A steam stripping process particularly useful for separating the volatile components of immiscible materials. In the disclosed embodiment of the invention peel oil emulsion, e.g., from a citrus juice extractor, is heated under pressure by the injection of steam while the emulsion is flowing. The major portion of the pressure on the flowing emulsion is then dropped through a back pressure valve, and the resultant vapor-liquid mixture is passed through a long turbulent passage under a slowly decreasing pressure where thorough mixing is provided and d-limonene, the volatile component of the peel oil emulsion, is stripped from the emulsion by the ad-mixed steam. The mixture is discharged at atmospheric pressure into a vapor collecting tank where the spent liquor is separated and drained off. The collected vapors are passed to a condenser, and the condensate therefrom is directed to a decant tank where the d-limonene is decanted off.

This is a division, of application Ser. No. 399,238 filed Sept. 20,1973, now U.S. Pat. No. 4,113,573.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to methods and apparatus for separating avolatile constituent from a liquid mixture, and more particularly, itpertains to methods and apparatus for steam stripping liquid mixtures ina continuous operation in order to extract a volatile constituentthereof.

2. Description of the Prior Art

Steam distillation, or steam stripping, is a well known process in whichvaporization of the volatile constituents of a charge of liquid materialis effected at a lowered temperature by the introduction of steamdirectly into the charge. Steam stripping is normally used where theconstituents to be separated are not sufficiently volatile to vaporizeunder normal controlled heating conditions. Typical steam strippinginstallations of the prior art include the vertically arranged packedcolumns or plate columns wherein the liquid feed material is insertedunder equilibrium conditions into the column where countercurrent flowprovides a continuous separation of the vapor and liquid phases of thematerial. The columns normally contain a plurality of rings, plates, orother forms of baffles so as to provide a maximum number of turbulenceproducing and mixing surfaces. Steam is bubbled up from the bottom ofthe tank to provide a thorough mixing and vapor-stripping of thematerial in the column, and the vapors are removed at the top of thecolumn while the spent liquor is continuously drained off at the bottomof the column. A duel-stage steam stripping apparatus for recoveringvegetable oils from an oil and solvent solution of hot miscella isshown, for example, in prior U.S. Pat. No. 3,503,854 to Good.

The aforementioned steam stripping apparatus and processes of the priorart, however, are not always ideally efficient where the material to beseparated comprises an emulsion and, particularly, where the volatileconstituent may be entrained in fine particulate matter in the emulsion.A typical example of an emulsion which cannot be readily separated withnormal steam stripping apparatus without long exposure times and theadditions of excessive amounts of steam is a citrus peel oil emulsion asobtained from a citrus juice extractor of the type shown, for example,in the prior U.S. patent to Robbins et al, U.S. Pat. No. 3,717,084. Inthe juice extracting machine shown in this patent, the juice isextracted from citrus fruit, such as oranges, by compressing the fruitbetween a pair of cups formed with a plurality of interdigitatingfingers. One of the cups is provided with a central tube to receive thejuice while the peel is scored and pushed through the fingers with thepeel oil being discharged therefrom in the process. Washing sprays areused to wash the oil from the peel and from the air, and the resultingpeel oil emulsion is directed away from the juice extracting machine.

The major constituent of the essential oil present in citrus fruit peelsis known as d-limonene and is useful in making dipentene resins such asturpentine. Approximately 95% of the essential oil in oranges, forexample, is d-limonene. With the juice extractor of the aforementionedUnited States patent to Robbins et al, a water-peel oil emulsion isproduced containing about 3-5% by volume of d-limonene with thed-limonene being immiscible in the water. In addition to the d-limonene,the emulsion also contains some coluble solids, mostly sugars andpectins, and insoluble solids in the form of fine pulp particles or bitsof peel.

Methods of recovering the d-limonene from the peel oil emulsion haveincluded centrifuging, but the amount of d-limonene recoverable by thistype of process is limited. Other prior art suggested methods forrecovering the essential oils from the citrus peel oil emulsion haveincluded various more or less conventional steam stripping operations asdisclosed, for example, in the U.S. Pat. No. to Pulley 2,471,893 or thearticle by Messrs. Veldhuis et al in the Journal of Food Science, Volume37, pages 108-112 (1972). In the patented steam stripping system citruswaste press water is heated to some 215° F. to 250° F., or higher, andis then passed through a holding chamber under sufficient pressure so asto prevent boiling with the retention time of the material in thechamber being about 10 seconds. The press water is flash discharged fromthe holding chamber into a flash tank through a spray nozzle. In theflash tank steam is introduced into the emulsion with the vapors beingcollected, condensed and directed to a decant tank where the essentialcitrus oil is separated from the water. In the process disclosed in theVeldhuis et al article, citrus waste water is directed into aconventional steam stripping column and the vapours therefrom aredirected into a reflux condenser and a chilling condenser with thecondensate therefrom being delivered to an oil separator.

Other prior art steam stripping processes which have been performed uponfruit juices for recovering volatile constituents thereof are disclosedin an article by A. H. Brown et al in Industrial & EngineeringChemistry, Volume 43, No. 12 (1951). In a process disclosed in thisarticle, the fruit juice material is heated and fed to a steam injectionheater. The resultant steam-liquid mixture is then directed to asix-foot, steam-jacketed tubular evaporator before being flashed into aflash tank where the vapor is separated from the liquid, condensed andseparated in the conventional manner.

SUMMARY OF THE INVENTION

By the present invention a unique process is provided which isparticularly useful for separating volatile constituents of immisciblematerials which are not otherwise readily separated by such distillationprocesses other than with long exposure times and with great excesses ofsteam. The method of the present invention generally comprises, in itspreferred form, the injection of live steam into the continuouslyflowing liquid mixture in order to raise the temperature of the mixturewhile it is under a relatively high pressure. The mixture is thendirected through a long confined passage with a plurality ofprotuberances therein while the pressure is being continuously decreasedin order to provide continuous vaporization and a very thorough mixingto achieve better separation of the volatile components of the mixture.In the preferred form of the invention a back pressure valve is usedbetween the steam injection means and the long turbulent passage inorder to provide rapid vaporization of the material and virtuallyexplode the various solid components in the mixture--thus aiding inreleasing the volatile constituents therefrom. At the end of theturbulent passage the mixture is flashed into a more or lessconventional vapor separation tank where the vapors are directed to acondenser and then to a decant tank for final water-oil separation.

While, in the preferred operation, live steam is injected into theliquid material to be separated it is possible to use one of thecomponents of the material as the vaporizable component which strips thevolatile component from the remainder of the material.

The aforedescribed steam stripping process has been found to beparticularly useful in separating d-limonene from a citrus peel oil orwaste water emulsion such as might be recovered from a citrus juiceextractor or from citrus peel oil presses.

With the method of the present invention d-limonene has been strippedfrom peel oil emulsion at or near theoretical steam consumption rates,and the percentage removal of the d-limonene from the emulsion has beenvery close to 100%. Furthermore, the recovered d-limonene issubstantially odor-free whereas other commercial d-limonene extractionprocesses produce a d-limonene product with a fruit odor or with adistinct burned odor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustration of the steam stripping processand apparatus of the present invention.

FIG. 2 is a diagrammatic representation of the steam stripping system ofthe present invention.

FIG. 3 is a diagrammatic isometric view of the vaporizing tank of thepresent invention with portions thereof being broken away for thepurpose of clarity.

FIG. 4 is an enlarged isometric detail view of a section of one of themixing conduits in the vaporizing tank of FIG. 3 with a portion thereofbeing broken away in order to illustrate the turbulence-producingstructure of the interior passage.

FIG. 5 is an enlarged partial vertical section taken through the lowerend of the vaporizing tank of FIG. 3.

FIG. 6 is an enlarged section through the steam-emulsion mixing valveshown in FIG. 2.

FIG. 7 is an enlarged section through the back pressure valve shown inFIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The process and apparatus of the present invention will be specificallydescribed with reference to the separation of d-limonene from citruspeel oil emulsion. However, it will be understood that the method andapparatus of the present invention may find utility in any other steamdistillation processes wherein a volatile constituent is to be separatedfrom liquid materials. The process of the present invention isparticularly useful wherein the material to be stripped is contained ina highly stable emulsion and wherein portions of the material to bestripped are contained within small particulate matter in the emulsion.Furthermore, while the process of the present invention will bedescribed as a steam stripping process wherein live steam is injectedinto the immiscible materials, it will be recognized that the use ofvaporizable carriers other than steam is possible. Furthermore, thesteam does not have to be injected but may exist in the immisciblematerials whereby the application of a sufficient amount of heat alonewill provide the vaporizing carrier for stripping another volatileconstituent from the material.

As pointed out, the process of the present invention has been found tobe particularly useful in separating d-limonene from a citrus peel oilemulsion. This process is shown diagrammatically in FIG. 1. The peel oilemulsion used in the process of the present invention was obtained froma citrus juice extractor 12 particularly designed to separately recoverpeel oil such as the extractor shown in the prior patent to Robbins etal, U.S. Pat. No. 3,717,084. In the device disclosed in this patentjuice extracting cups compress and score the peel of the citrus fruitand force it through narrow passageways in the cups. The juice and peelare separately recovered (as shown in FIG. 1). During the aforedescribedoperations on the peel the peel oil is released from the peel cells, andwash sprays are used to remove the expelled peel oil from the atmosphereand to wash it from the peel and pulp. This peel oil-water emulsionmaterial is then screened to provide the base material from which thed-limonene is to be extracted. With the process of the present inventionthe peel oil emulsion is passed through screens having performations of0.020 inch dimensions before being directed to the separation apparatus.

The cold peel oil emulsion, after screening, is passed to a pump 14(FIG. 1) having sufficient capability to provide the necessary backpressure on the downstream end thereof as will be explained in greaterdetail hereinafter. The liquid material from the pump 14 is next passedthrough a heat exchanger 16 wherein a sufficient amount of heat is addedto the material to bring the temperature thereof up to around 200°-214°F. Immediately after the material is heated in the heat exchanger, livesteam is injected therein by a mixing valve 18 to add additional heatthereto to bring the temperature of the emulsion up to about 270° F.This emulsion is then passed through a back pressure valve 20 where thepressure thereof is dropped from about 70 psig to about 15 psig. Thisdrop in pressure instantly vaporizes the steam in the emulsion and hasan explosive effect upon the small particles of peel and pulp in theemulsion so as to aid in allowing the entrained d-limonene constituentto be stripped therefrom.

The vapor stripping operation basically occurs in a mixing tube 22 whichprovides a long confined turbulent passage in which the steam has aresidence time of about five seconds. The liquid material, which clingsto the sidewalls of the passage, has a somewhat longer residence time.The particular structure and function of this mixing tube will bedescribed in greater detail hereinafter. During passage through themixing tube the superatmospheric pressure on the emulsion and ad-mixedsteam is reduced from about 15 psig to atmospheric pressure which is thepressure existing in a vapor separation tank 24 at the outlet of thepassage through the mixing tube. The vapors in the tank 24, at atemperature of about 214° F., are collected and directed to a condenser26 which condenses them and directs the condensate to a decant tank 28,both the condenser and decant tank being entirely conventional items intypical commercial steam stripping systems. In the decant tank thelighter d-limonene constituent is allowed to separate from the heavierwater constituent and is decanted off.

The material remaining in the liquid phase in the vapor separation tank24 is drained off to a pump 30 which pumps it through the heat exchangerunit 16 thus providing the heat for the initial heating of the coldemulsion. After passing through the heat exchanger the spent emulsionmay be discarded.

The specific apparatus for accomplishing the process of the presentinvention is diagrammatically shown in FIG. 2. As pointed outpreviously, the emulsion is passed from the pump 14 through the heatexchanger 16 to the mixing valve 18 at which point the temperaturethereof should be in a range of from about 200°-214° F. depending uponthe efficiency of the heat exchanger and the heat losses occurringduring transport of the spent emulsion from the vapor separation tank 24to the heat exchanger. At the mixing valve, the emulsion is mixed withhigh pressure steam. The structure of the mixing valve is shown indetail in FIG. 6 wherein it will be seen that the heated emulsionproduct is passed from an inlet pipe 31 to an outlet pipe 32 in astraight line flow via a slightly restricted passage through a nozzlemember 34. This cylindrical member is welded at its upstream end to afitting 31a to which the pipe 31 is threaded. Steam from a steam inletline 33 is injected into an enlarged housing 35 of the mixing valvewhich surrounds the nozzle member 34. From this housing the steam isinjected into the pipe 32 through a small annular passage 36 surroundingthe outlet end of the cylindrical nozzle member. Thus, the steam isinjected tangentially about the product flow.

As shown in FIG. 2, the steam inlet line 33 is provided with a controlvalve 38 which is an entirely conventional pneumatically operatedprocess control valve serving to limit the amount of steam injected intothe emulsion. This pneumatically operated valve is controlled by thetemperature of the emulsion at the downstream end of the mixing valve 18which temperature is sensed by a temperature controller 39 at the upperend of outlet pipe 32 just ahead of the back pressure valve 20. Inpracticing the process of the present invention with a citrus peel oilemulsion in the system the temperature sensed by controller 39 should bemaintained generally in the range of from about 270° F. to about 275° F.although acceptable results have been obtained with the temperaturebeing in a range of from about 250° F. to about 280° F. The steam, atabout 150 psig, is directed through the steam inlet control valve 38into the mixing valve 18, and, at the heretofore mentioned temperatureand pressure of the peel oil emulsion in the pipe 32, the steam will becondensed so that a totally liquid mixture is provided at the backpressure valve 20. While this condensation of the steam is notabsolutely necessary, it is useful since it provides a means forachieving control of the pressure and temperature conditions prior tothe back pressure valve--such conditions being critical in achieving thedesired stripping results.

A check valve 40 is provided just downstream of the heat exchanger 16 onthe inlet pipe 31 the function of which is to prevent the steam in thesteam inlet line 33 from driving the pump out of control in the reversedirection in the event that there is a pump failure.

The back pressure valve 20 provides an important function in theapparatus of the present invention by rapidly reducing the pressure ofthe steam-emulsion mixture to vaporize the steam component thereof andinitiate the stripping operation. The structure of the back pressurevalve is shown in detail in FIG. 7 wherein it will be seen that themixture is directed from an inlet conduit 41 to an outlet conduit 42through a passage in a valve plate 44 which is adapted to be closed by avalve plug element 46. The valve plug element is mounted upon acompression spring 48 which provides about 150 psi compression tomaintain the plug element in the valve seat unless it is unseated by anoperator 50. This compression spring 48 also acts as a relief spring toallow the valve to open under a predetermined upstream pressure of 150psi in the event that the operator 50 is malfunctioning. Theconventional diaphragm operator 50 includes an outer chamber 52 having ahydraulically actuated diaphragm therein (not shown). The diaphragm iscounterbalanced by a compression spring 54 which is set at the pressureat which the material on the upstream side of the back pressure valve isto be maintained. In the described system this pressure is ideally about70 psig although useful results have been obtained with the pressure aslow as 50 psig. The compression spring 54 is mounted upon a platform 55which is rigidly secured to the frame of the operator by a pair ofsupport rods 56 (one only being shown in FIG. 7). Attached to thediaphragm and extending longitudinally through the operator is anoperating stem 58 which, in the unactuated or neutral position of theoperator, is adapted to just clear the end of the valve plug element 46,as is shown in FIG. 7. The hydraulic pressure side of the diaphragm isconnected to a small pressure line 60 which is, in turn, connected tothe emulsion inlet pipe 31 (FIG. 2) at the check valve 40. This line 60thereby senses the upstream pressure on the back pressure valve 20 andmaintains this pressure at a predetermined value as set by the amount ofcompression in the spring 54. As shown in FIG. 7, this compression maybe adjusted by loosening the nuts 62 and adjusting the position of thereaction platform 55 upon the support rods 56. The pressure sensing line60 is connected to the emulsion inlet line upstream of the steam inletline 33 so that the fluxuating pressures caused by the injected steam donot adversely affect the control at the back pressure valve.Furthermore, an air ballast tank 64 (FIG. 2) is provided in the pressuresensing line 60 to further smooth out any pressure variations thereinwhich might be particularly due to the pulsating steam injection. Agauge 66 is attached adjacent to the ballast tank so that this upstreampressure, which is critical to the operation of the process, can bereadily checked.

An important feature of the apparatus of the present invention is avaporizing tank 70 to which the emulsion-steam mixture is directed. Thisvaporizing tank 70 comprises both the mixing tube 22 and the vaporseparation tank 24 shown schematically in FIG. 1. The structure of thevaporizing tank is shown in detail in FIG. 3 and FIG. 5. From FIG. 3 itwill be seen that the mixture in conduit 42 from the back pressure valve20 is directed to a manifold 72 where it is split into six separateportions and directed outwardly into six mixing pipes 74 to the outerportion of the tank. The pipes 74 collectively comprise the mixing tube22 portion of the apparatus, and each of these pipes is angleddownwardly at the sidewall of the tank and includes a relatively long,continuous forty-foot flow section comprised of a plurality ofend-to-end interconnected, parallel arranged vertically extendingsegments 74a. The vapor separation tank 24 is comprised of a separatetank located inside of and spaced from the walls of the vaporizing tank70, as shown in FIG. 3. It should be noted that the dimensions of thevaporizing tank 70 are approximately 10 feet high and 5 feet wide in theillustrated embodiment of the invention, and hence, the 40 footdimension of each of the pipes 74 is relatively long in contrast withthe dimensions of the vaporizing tank or the vapor separation tank whichis contained therein. The lower end of each of the mixing pipes 74includes a short horizontally extending segment 74b the outlet end ofwhich is directed into the lower end of the vapor separation tank. Theliquid-vapor mixture which is injected into the separation tank 24includes vapors which pass under a cylindrical baffle 76 (FIG. 5) andare directed upwardly to an outlet pipe 78 for passage to the condenser26. The spent emulsion is received on the lower downwardly sloping walls24a of the tank 24 and is drained by a vertical discharge line 80. Theouter wall of the vaporizing tank 70 is provided with insulation 84 toprevent heat losses from the material being processed therein.

As pointed out hereinbefore, a special feature of the present inventionis the mixing tube 22 which is comprised of the several separateconduits 74. Each conduit comprises a 21/2 inch O.D. pipe with a 23/8inch I.D. and is formed with a series of interconnected verticalsegments 74a to provide a relatively long passage for the emulsion fromthe manifold 72 to the outlets at the lower end of the tank 24. Each ofthe conduits 74 thereby restricts the flow and further includes aninterior coil 88 formed of 1/8 inch wire and positioned in engagementwith the cylindrical walls of the conduit passage as shown in FIG. 4.These coils provide considerable turbulence in the conduits and aid inachieving a complete mixing of the steam with the emulsion during thestripping phase of the process.

As stated previously the collected vapors from the separation tank 24are directed out the pipe 78 to the condenser 26 which may be aconventional shell and tube condenser provided with continuouslycirculating cooling water as shown in the diagrammatic illustration ofFIG. 2. The condensate from the condenser is directed through a verticaldischarge pipe 90 to a decant tank 28 wherein the light d-limonenefraction is decanted out.

Further as shown in FIG. 2, the spent emulsion is directed from thedischarge line 80 to the pump 30 which pumps it through the heatexchanger 16 before it is discharged in a line 92. The hot spentemulsion from the vaporizing tank 70 thereby provides the heat forheating the cold peel oil emulsion prior to injecting the steam thereinand thus increases the efficiency of the operation.

As an example of the results obtained by the aforedescribed process andapparatus when treating orange peel oil emulsion for the extraction ofd-limonene, attention is directed to Table I which provides, in tabularform, the results of 17 separate tests performed while the process wasin continuous operation using orange peel oil emulsion as provided byorange juice extractors of the type disclosed in the aforementioned U.S.patent to Robbins et al. It can be seen that with a controllertemperature in the 270°-275° F. range a recovery of 99% of thed-limonene in the emulsion was achieved with a steam/d-limonene ratio ofabout 12 to 1. In Test No. 5, for example, with a feed rate of 302 lbs.of emulsion/min., a spent liquor rate of 339 lbs./min., and a steam rateof 65 lbs./min., 21.8 lbs./min. of condensed water was obtained and 5.8lbs./min. of d-limonene. The weight ratio of water to d-limonene wasnotably low at 3.8, and the d-limonene recovery was 94% measured aspercent recovery from the effluents.

From the foregoing description it will be seen that the process andapparatus of the present invention provide a highly efficient method forsteam stripping the volatile constituent of immiscible materials with aminimum amount of steam and in a minimum period of time. The apparatusis relatively simple and contains few expensive control devices. Suchapparatus is therefore capable of being constructed at a low cost andwill be seen to require only a minimum amount of maintenance in theoperation thereof.

Although the best mode contemplated for carrying out the presentinvention has been herein shown and described, it will be apparent thatmodification and variation may be made without departing from what isregarded to be the subject matter of the invention.

                                      TABLE I                                     __________________________________________________________________________                           Weight Ratio  Average                                                         of Condensed                                                                          Average                                                                             d-Limonene                                               Steam  Water to d-                                                                           d-Limonene                                                                          Content of                                         Oil Emul-                                                                           ratio  Limonene                                                                              Content of                                                                          Spent Approximate                           Controller                                                                           sion Feed                                                                           (Lb. Steam/                                                                          Collected                                                                             Feed  Liquor                                                                              % Recovery of                      Test                                                                             Temperature                                                                          Rate  Lb. d- (Lb. Water/Lb.                                                                        (Percent                                                                            (Percent                                                                            d-Limonene                         No.                                                                              (°F.)                                                                         (Gal./Min.)                                                                         Limonene)                                                                            d-Limonene)                                                                           W/W)  W/W)  (Percent)                          __________________________________________________________________________    1  245-250                                                                              37    12.3   3.8     2.6   0.27  85                                 2  255-260                                                                              46    10.3   2.7     2.1   0.20  90                                 3  270-275                                                                              36    11.0   3.8     2.2   0.08  94                                 4  270-275                                                                              36    10.0   3.3     2.0   0.09  96                                 5  270-275                                                                              36    11.2   3.8     1.9   0.10  94                                 6  250    28    9.2    2.3     2.3   0.27  88                                 7  255    25    7.8    1.6     2.3   0.29  88                                 8  270-275                                                                              38    7.7    2.6     2.8   0.14  95                                 9  270-275                                                                              36    7.3    2.5     3.0   0.11  96                                 10 270-275                                                                              32    8.2    2.9     2.7   0.12  95                                 11 270-275                                                                              32    8.2    2.9     2.7   0.16  94                                 12 270-275                                                                              32    12.8   4.4     1.9   0.04  99                                 13 250-285                                                                              31    14.1   4.8     1.3   0.02  99                                 14 270-275                                                                              36    12.5   4.1     1.6   0.02  99                                 15 265-270                                                                              41    12.8   4.3     1.5   0.03  99                                 16 275-280                                                                              38    12.4   4.4     1.9   0.02  99                                 17 275-280                                                                              38    10.8   3.7     2.4   0.04  99                                 __________________________________________________________________________

I claim:
 1. In a process for distilling a volatile constituent from aliquid mixture of immiscible materials, the steps of heating saidmixture to a temperature in the range of from about 250° F. to about275° F. while said mixture is in a flowing state and at asuperatmospheric pressure, substantially and abruptly reducing the majorportion of the superatmospheric pressure of said mixture to rapidlyvaporize a constituent of said mixture, immediately thereafterconducting said mixture and vaporized constituent through a relativelylong turbulent passage while simultaneously continuously reducing thepressure to provide heat for further vaporizing a constituent of saidmixture, reducing the pressure of said mixture to atmospheric pressureat the outlet end of said turbulent passage and separating the vaporsfrom the remaining liquid, condensing said vapors to obtain acondensate, and separating said volatile constituent from the carrierconstituent in said condensate.
 2. In a process according to claim 1wherein the pressure of said mixture is reduced to a value in the rangeof from about 10 psig to about 15 psig prior to the conduction thereofthrough said turbulent passage.